A night vision system converts available low intensity ambient light to a visible image. These systems require some residual light, such as moon or star light, in which to operate. This light is generally rich in infrared radiation, which is invisible to the human eye. The ambient light is intensified by the night vision scope to produce an output image which is visible to the human eye. The present generation of night vision scopes use image intensification technology and, in particular, microchannel plates to amplify the low level of visible light and to render a visible image from the normally invisible infrared radiation. The image intensification process involves conversion of the received ambient light into electronic patterns and the subsequent projection of the electron patterns onto a receptor to produce an image visible to the eye. Typically, the receptor is a phosphor screen which is viewed through a lens provided as an eyepiece.
Specific examples of microchannel plate amplification are found in the image intensifier tubes of the night vision devices commonly used by police departments and by the military for night time surveillance, and for weapon aiming. However, microchannel plates may also be used to produce an intensified electrical signal indicative of the light flux or intensity falling on a photocathode, and even upon particular parts of the photocathode. The resulting electrical signals can be used to drive a video display, for example, or be fed to a computer for processing of the information present in the electrical analog of the image.
In night vision devices, a photoelectrically responsive photocathode element is used to receive photons from a low light level image. Typically the low light level image is far too dim to view with unaided natural vision, or may only be illuminated by invisible infrared radiation. Radiation at such wavelengths is rich in the nighttime sky. The photocathode produces a pattern of electrons (hereinafter referred to as "photoelectrons") which correspond with the pattern of photons from the low-level image. Through the use of electrostatic fields, the pattern of photoelectrons emitted from the photocathode are directed to the surface of a microchannel plate.
The pattern of photoelectrons is then introduced into a multitude of small channels (or microchannels) opening onto the surface of the plate which, by the secondary emission of electrons, produce a shower of electrons in a pattern corresponding to the low-level image. That is, the microchannel plate emits from its microchannels a proportional number of secondary emission electrons. These secondary emission electrons form an electron shower thereby amplifying the electrons produced by the photocathode in response to the initial low level image. The shower of electrons, at an intensity much above that produced by the photocathode, is then directed onto a phosphorescent screen. The phosphor of the screen produces an image in visible light which replicates the low-level image. Understandably, because of the microchannel plate, the representative image is pixelized, or is a mosaic of the low-level image.
More particularly, the microchannel plate itself conventionally is formed from a bundle of very small cylindrical tubes which have been fused together into a parallel orientation. The bundle is then sliced to form the microchannel plate. These small cylindrical tubes of the bundle thus have their length arranged generally along the thickness of the microchannel plate. That is, the thickness of the bundle slice or plate is not very great in comparison to its size or lateral extent; however, the microchannels individually are very small so that their length along the thickness of the microchannel plate is still many times their diameter. Thus, a microchannel plate has the appearance of a thin plate with parallel opposite surfaces.
The plate may contain millions of microscopic tubes or channels communicating between the faces of the microchannel plate. Each tube forms a passageway or channel opening at its opposite ends on the opposite faces of the plate. Further, each tube is slightly angulated with respect to a perpendicular from the parallel opposite faces of the plate so that electrons approaching the plate perpendicularly can not simply pass through one of the many microchannels without interacting with the interior surfaces.
Internally the many channels of a microchannel plate are each defined by or are coated with a material having a high propensity to emit secondary electrons when an electron falls on the surface of the material. In addition, the opposite faces of the microchannel plate are conventionally provided with a conductive metallic electrode coating so that a high electrostatic field can be applied across the plate. As previously indicated, an electrostatic field is also applied between the photocathode and the microchannel plate to move the photoelectrons emitted by the photocathode to the microchannel plate. Consequently, electrons produced by the photocathode in response to photons from an external image travel to the microchannel plate in an electron pattern corresponding to the received pattern of low-level light. These electrons enter the channels of the microchannel plate and strike the angulated walls which are coated with the secondary electron emissive material. Thus, the secondary emission electrons in numbers proportional to the number of photoelectrons, exit the channels of the microchannel plate to impinge on a phosphorescent screen. An electrostatic field between the microchannel plate and the phosphor screen accelerates the electrons to the screen producing an intensified mosaic image of the low-level scene.
Rather than directing the electron shower from a microchannel plate to a phosphorescent screen to produce a visible image, the shower of electrons may be directed upon an anode in order to produce an electrical signal indicative of the light or other radiation flux incident on the photocathode. The electrical analog signal may be employed to produce a mosaic image by electrical manipulation for display on a cathode ray screen, for example. Still alternatively, such a microchannel plate can be used as a "gain block" in a device having a free-space flow of electrons. That is, the microchannel plate provides a spatial output pattern of electrons which replicates an input pattern, and at a considerably higher electron density than the input pattern. Such a device is useful as a particle counter to detect high energy particle interactions which produce electrons.
Regardless of the data output format selected, the sensitivity of the image intensifier or other device utilizing a microchannel plate is directly related to the amount of electron amplification or "gain" imparted by the microchannel plate. That is, as each photoelectron enters a microchannel and strikes the wall, secondary electrons are knocked off or emitted from the area where the photoelectron initially impacted. The physical properties of the walls of the microchannel are such that, generally, a plurality electrons are emitted each time these walls are contacted by one energetic electron. In other words, the material of the walls has a high coefficient of secondary electron emission or, put yet another way, the electron-emissivity of the walls is greater than one.
Propelled by the electrostatic field across the microchannel plate, the secondary electrons travel toward the far surface of the microchannel plate away from the photocathode and point of entry. Along the way, each of the secondary electrons repeatedly interact with the walls of the microchannel plate resulting in the emission of additional electrons. Statistically, some of the electrons are absorbed into the material of the microchannel plate so that the photoelectrons do not generally escape the plate. However, the secondary electrons continue to increase or cascade along the length of the microchannels. These electrons in turn promote the release of yet additional electrons farther along the microchannel tube. The number of electrons emitted thus increases geometrically along the length of the microchannel to provide a cascade of electrons arising from each one of the original photoelectrons which entered the tube. As discussed above, this electron cascade then exits the individual passageways of the microchannel plate and, under the influence of another electrostatic field, is accelerated toward a corresponding location on a display electrode or phosphor screen. The number of electrons emitted from the microchannel, when averaged with those emitted from the other microchannels, is equivalent to the theoretical amplification or gain of the microchannel plate.
While the intensity of the original image may be amplified several times, various factors can interfere with the efficiency of the process thereby lowering the sensitivity of the device. For example, one inherent problem of microchannel plates is that a photoelectron released from the photocathode may not fall into one of the slightly angulated microchannels but impacts the bluff conductive face of the plate in a region between the openings of the microchannel tubes. Electrons that hit the metallized conductive face are likely to be deflected or bounce back toward the photocathode before being directed back to the microchannel plate by the electrostatic field. Such bounced photoelectrons, which then produce a number of secondary electrons from a part of the microchannel plate not aligned with the proper location of photocathode generation, decrease the signal-to-noise ratio, visually distorting the image produced by the image intensifier. Other times the errant electron is simply absorbed by the metallized conductive face of the plate and is not amplified to produce part of the image or signal produced by the detector anode.
Of course, one solution to this problem is to increase the amount of microchannel aperture area on the input face of the microchannel plate as was done in U.S. Pat. No. 4,737,013, issued 12 Apr. 1988, to Richard E. Wilcox. Through the use of an etching barrier around each microchannel, these particular microchannel plates have an improved ratio of total end open area of the microchannels to the area of the plate. Specifically, the etching barrier incorporated in the plate allows more precise etching of the microchannel tubes in the plate. The technique allows the plates to be produced with a theoretical open area ratio (OAR) of up to 90% of the plate active surface. As a result, the photoelectrons are not as likely to miss one of the microchannels and impact on the face of the microchannel plate to be bounced into another one of the microchannels. This higher OAR improves the signal-to-noise ratio of image intensification.
While the OAR may be improved using conventional methods, other factors still reduce the gain and decrease the signal-to-noise ratio of the conventional microchannel plate. In particular, coating the input face of the conventional microchannel plates with a conductive metallic electrode material significantly reduces the gain provided by a microchannel plate. Generally, the conductive metallic electrode materials on a statistical basis have an electron-emissivity coefficient of less than unity (i.e., less than one). More particularly, conventional deposition procedures for these metallic electrodes entail rotationally disposing the microchannel plate so that the axis of the microchannels is parallel to an axis about which the microchannel plate may be rotated. A deposition source is angularly disposed relative to the axis of the microchannels at a distance from the input face of the plate. As the microchannel plate is rotated in a high vacuum, metallic material is evaporated from the source onto the microchannel plate.
Because the microchannel plate rotates about an axis which is parallel with the axis of the microchannels, the metallic material from the source coats not only onto the input face of the microchannel plate, but also for a distance into the microchannels themselves. The distance into the microchannels to which the metallic electrode material will coat is dependent upon the angulation of the source with respect to the axis of the microchannels themselves. Because the microchannel plate is rotated about an axis parallel to the axis of the microchannels during deposition of the metallic electrode coating, the depth of metallic coating penetration into the microchannels is substantially uniform circumferentially about the microchannels. That is, the angulation of the microchannels relative to the evaporation source is held constant to produce uniform depth of penetration of the metallic electrode coating into the channels.
As a result, in addition to covering the face of the microchannel plate, the conductive electrode coating extends into the individual microchannels of the plate, covering a substantial part of the entrance surface portion of each microchannel which would be visible (on a microscopic scale) if one were to look into the microchannels perpendicularly to the face of the plate. Accordingly, while conventional processes and methods for deposition of the metallic conductive electrode material renders the parallel faces of the microchannel plates sufficiently conductive, the unavoidable coating of the inner entrance portion surfaces of the microchannels themselves unavoidably interferes with amplification of photoelectrons due to the low electron-emissivity coefficient of the coating material.
Moreover, the deposition of the coating material, performed in a vacuum under very exacting conditions and requiring specialized fixtures, is the single-most expensive manufacturing step in the production of conventional microchannel plates
Typical microchannel plate coating materials are metallic and have a electron-emissivity coefficient of less than unity (i.e., less than 1). That is, a photoelectron striking the metallic conductive coating will not release more than one secondary electron as it is absorbed. Statistically, these metallic conductive electrode materials have an electron-emissivity of about 0.8. Accordingly, about twenty percent of the photoelectron signal that hits the metallized surface of the microchannel plate is immediately lost to the conductive electrode coating within the entrance portion of the microchannels. This lost signal value can not be amplified in the microchannel plate, and cannot contribute to output from the microchannel plate. Thus, the sensitivity of the microchannel plate is decreased.
More particularly, an electron, whether a photoelectron or secondary electron, which strikes the conductive metallic electrode coating may be absorbed without releasing any subsequent electrons from the material. As such, when an electron emitted from a photocathode strikes the conductive coating on the surface of a microchannel, there is no initial amplification and the electron may be absorbed without resulting in the emission of even a single secondary electron. This secondary electron, if it were emitted, could be multiplied subsequently in the microchannel and would contribute to the output of the microchannel plate. With no amplification by the emission of secondary electrons in the entrance portion of the microchannel plate, the microchannels have essentially been shortened by the length covered with the conductive electrode coating. No amplification by emitted secondary electrons will occur until the initial photoelectron (or its substitute) passes beyond the conductive material deposited in the microchannel.
However, the solution to this problem is not as simple as simply increasing the length of the microchannels so as to extend the length over which the secondary electron emission process is effective. At first blush, it would seem that the gain of a microchannel plate could be increased indefinitely simply by making the plate thicker. However, a microchannel plate cannot simply be made thicker because doing so severely and adversely affects the signal-to-noise ratio of the microchannel plate. The reason for this prohibition against increasing the thickness of a microchannel plate to increase its gain can be understood when one considers the statistical effects involved in emission of secondary electrons within the microchannels.
Each time an electron impacts the wall of a microchannel, there is a probability of the electron causing the emission of one or more secondary electrons. For the metallic electrode material, which is on the entrance portions of the microchannels of conventional microchannel plates, this probability coefficient is about 0.8. Thus, there is some electron signal loss and loss of amplification length for the microchannel plate because of this metallic electrode material at the entrance portion of the microchannels. For the material along the remaining length of the microchannels, the secondary electron-emissivity is greater than one, and the statistical process results in an increase in the number of electrons moving along the channels from the entrance end to outlet end. However, each time an electron impacts the walls of a microchannel, there is also the statistical probability that a positive ion will be released. When a positive ion is released, it travels in the opposite direction to the electron flow along the microchannel because of the prevailing electrostatic field. As a positive ion travels toward the entrance end of a microchannel, it also will impact and interact with the walls of the channel. Similarly to an electron, a positive ion has a probability of causing emission of secondary electrons.
Secondary electrons which are emitted because of positive ions moving toward the inlet end of a microchannel plate represent noise in the output of the microchannel plate. A point of diminishing returns is reached if a microchannel plate is increased in thickness beyond a certain length-to-diameter ratio for the microchannels. Further increase in the thickness of the microchannel plate results in little or no increase in gain because of space-charge saturation. If the voltage across the microchannel plate is increased to overcome the space-charge saturation limit, the probability of emission of positive ions increases faster than the emissivity of electrons. As a result, the signal-to-noise ratio of the thicker microchannel plate is severely decreased.
Accordingly, it is an object of the present invention to provide an improved microchannel plate having both increased electron-emission gain and an improved signal-to-noise ratio.
Another object for this invention is to provide such an improved microchannel plate which does not require the application of a metallic electrode coating to an active microchannel area of the plate.
It is yet another object of the present invention to provide an image intensifier tube which incorporates such an improved microchannel plate.